The faceplate for a cathode ray tube of the type used in television picture tubes is fabricated from a transparent glass exhibiting a matrix of physical properties mandated by the process for manufacturing cathode ray tubes. The glass must also demonstrate a high degree of X-radiation absorption and resist discoloration (termed "browning" in the art) caused by the bombardment of high velocity electrons thereupon and exposure to X-radiation.
The concerns regarding X-radiation absorption have increased as voltages applied at the anode have been raised by the television picture tube manufacturers. Thus, as the voltage at the anode is raised, the electron beam generated from the cathode is further accelerated. The beam strikes and penetrates the aluminum film, the phosphor screen, and the faceplate of the tube emitting X-radiation. As can be appreciated then, the higher the voltage applied, the greater the need for X-radiation absorption by the faceplate glass. Hence, whereas in the past a glass possessing an X-radiation absorption coefficient of 26 cm.sup.-1 at a wavelength of 0.6 Angstroms was deemed to be quite satisfactory, it is now adjudged that the coefficient at 0.6 Angstroms should be raised to at least 28 cm.sup.-1.
In recent years, the trend has been to use higher and higher anode voltages in commercial television tubes. In fact, some tubes now use voltages of 40 KV or higher. As the voltage increases, the short wavelength end of the X-radiation range decreases. To illustrate, the short wavelength limit for 30 KV is 0.41 Angstroms; that for 40 KV is 0.31 Angstroms. Thus, it now becomes desirable to have the absorption as high as possible at 0.3 Angstroms, while still maintaining the industry standard of a minimum linear absorption coefficient (.mu.) of 28 cm.sup.-1 at 0.6 Angstroms. A linear absorption coefficient of at least 8.5 cm.sup.-1 at 0.3 Angstroms is desirable.
Although the utility of lead oxide in absorbing X-radiation is well known to the art, the presence of lead oxide in a glass may result in permanent "browning" of the glass when it is subjected to electron bombardment. De Gier et al., in U.S. Pat. No. 2,477,329, observed that browning of a glass resulting from the bombardment of electrons could be essentially avoided by eliminating the presence of readily reducible oxides, particularly lead oxide, from the glass composition. The patent also notes that such discoloration of glasses by X-radiation could be reduced by including CeO.sub.2 in the glass composition.
U.S. Pat. No. 3,464,932 (Connelly et al.) discusses alternative means for obtaining glasses displaying high X-radiation absorption coefficients. The patent observes that the final value demonstrated by a glass results from a combination of the mass absorption coefficients of the individual constituents of the glass composition. Hence, to secure glasses exhibiting desired high X-radiation absorption coefficients requires the use of components possessing high mass absorption coefficients. The patent is particularly concerned with the relative merits of Sr0 and Ba0 for absorbing X-radiation in the wavelength range of interest between 0.35 and 0.77 Angstroms.
However, as was noted above, in addition to providing X-radiation absorption, and being resistant to discoloration from impingement of X-radiation and high velocity electrons, the faceplate glass must also satisfy an extensive matrix of physical properties. To illustrate:
First, the glass must exhibit a linear coefficient of thermal expansion (25.degree.-300.degree. C.) between 97-100.times.10.sup.-7 /.degree. C.;
Second, the glass must possess an annealing point not lower than about 500.degree. C., and a strain point not lower than about 455.degree. C.;
Third, the glass must demonstrate an electrical resistivity (log R) greater than 9 at 250.degree. C. and greater than 7 at 350.degree. C., and
Fourth, the glass must exhibit a viscosity at the liquidus temperature of at least 100,000 poises (10,000 Pa.s).
In recent years, the glass manufacturer has been confronted with ever increasing restrictions on the use of materials contributing to environmental pollution. For example, a fluoride-containing material has frequently been employed to provide an additional fluxing agent to improve the melting capability of a glass. Also, arsenic has traditionally been utilized to eliminate bubbles that occur in a glass during the melting process. Unfortunately, volatilization of fluorine and arsenic during the batch melting process results in air pollution problems. Consequently, the glass manufacturer has moved to eliminate the use of arsenic and fluoride-containing materials as batch materials.
Glass compositions for the fabrication of faceplates for television picture tubes essentially free of lead, arsenic and fluorine have been, and are, marketed commercially. In order to maintain the necessary level of X-radiation absorption, zirconia has commonly been incorporated in these glass compositions in substitution for lead.
Unfortunately, these zirconia-containing glasses have been found to be much more difficult to melt. The removal of lead oxide and fluorine retards the rate of melting. The removal of arsenic oxide decreases the fining rate, thus requiring a longer time to render the glass free of gaseous inclusions. Also the solution of zircon (batch source of zirconia) is slow. This gives rise to unmelted batch as well as an increase in the number of gaseous inclusions in the glass.
As a result, in order to obtain high quality glass, it becomes necessary to either increase the melting temperature of the glass, or increase the time at which the glass is held at the melting temperature. Either of these measures decreases the amount of glass that can be produced over a given period of time. As can be appreciated, both alternatives increase the overall production costs of the glass.